Array antennas and particularly phased controlled array antennas have become increasingly attractive, not only for military applications but also for civil and commercial applications. Array antennas can be advantageously utilized in radar systems, in radio telescopes or in so-called base stations in a wireless telecommunication network etc. One of the most favourable properties of an array antenna and particularly a phased controlled array antenna is the increased ability to dynamically and very quickly re-forming and/or re-directing the antenna lobe.
In particular, this can be utilized to avoid transmitting and/or receiving interference signals to and from neighbouring transmitters and/or receivers. In many cases the antenna lobe can be formed and/or directed to avoid receiving and/or transmitting such disturbances. In radar systems this ability can e.g. be used to avoid hostile jamming sources. In cellular telecommunication system or similar this ability can e.g. be used to enhance the utilization of the available frequency spectrum, e.g. the frequency spectrum in a GSM-system, a CDMA-system, a WCDMA-system or other similar radio communication systems. This is only examples of applications. There is a vast spectrum of different applications, as is well-known.
The ability to dynamically and very quickly re-forming and/or re-directing the antenna lobe is also advantageous in that the antenna lobe can be directed to transmit and/or receive electromagnetic radiation to and/or from a small geographical area, which increases the energy efficiency of the antenna system. These and other advantages provided by array antennas and particularly by phased controlled array antennas are well-known in the art of array antennas and they need no further explanation.
An array antenna is basically a spatially extended collection of several substantially similar antenna elements. The expression “spatially extended” implies that each element has at least one neighbouring element that is placed at a close distance so as to avoid emission of electromagnetic radiation in ambiguous directions. The expression “similar” implies that preferably all elements have the same polar radiation patterns, orientated in the same direction in 3-d space. However, the elements do not have to be spaced on a regular grid, neither do they have to have the same terminal voltages, but it is assumed that they are all fed with the same frequency and that one can define a fixed amplitude and phase angle for the drive signal of each element.
By adjusting the relative phases of the respective signals feeding the antenna elements in an array antenna the effective radiation pattern (the antenna lobe) of the antenna can be reinforced in a desired direction and suppressed in undesired directions. The relative amplitudes of, and constructive and destructive interference effects among, the signals radiated by the individual antenna elements determine the effective radiation pattern of the array antenna. An ordinary array antenna can be used to accomplish a fixed radiation pattern (fixed antenna lobe), whereas a more sophisticated phase controlled array antenna can be used to rapidly scan the radiation pattern (the antenna lobe) in azimuth and/or elevation.
However, depending on the individual antenna elements chosen for the array antenna in question there is formally at least one direction in which the antenna lobe cannot be readily directed, i.e. there is at least one null point.
The individual antenna elements in an array antenna can e.g. be the well-known dipole 10 or similar, as schematically illustrated in FIGS. 1A-1D. The exemplifying dipole 10 in FIG. 1A comprises two opposite radiating elements 11a, 11b. The radiating elements 11a, 11b are preferably shaped as elongated threads, cylinders or rectangles so as to extend ¼ (λ/4) of the utilized wavelength along a horizontal axis DP1. Each radiating element 11a, 11b is individually connected to a feeding line 12a, 12b in a well-known manner for communicating high frequency signals to and from the dipole 10. Hence, formally the dipole 10 comprises two ports. One usually considers the balanced (or differential mode) current Idiff=(I1−I2)/2 to be the current that excites the dipole, where the power conveyed by Idiff is supposed to convert to transmitted electromagnetic power. The differential mode is illustrated in FIG. 1A by a first current I+ fed to the first feeding line 12a (the first port) and a second current I− fed to the second feeding line 12b (the second port). The two currents I+, I− are of substantially equal magnitude but provided with opposite suffixes to indicate that they are out of phase by 180°, i.e. to indicate that the dipole 10 is operating according to a balanced or differential mode in a well-known manner. Balanced dual port dipole antennas like this have been studied extensively and can be made broadband and also scannable to a fair extent.
FIG. 1B illustrates a cross-section of a schematic radiation pattern from the dipole 10 cut along the axis DP1, and FIG. 1C illustrates a top view of said schematic radiation pattern, whereas FIG. 1D illustrates a schematic perspective view of the radiation pattern in FIGS. 1B-1C. As can be seen there is substantially no radiation emanating along the axis DP1, i.e. there is substantially no radiation from the short ends of the radiating elements 11a, 11b. This implies that an array antenna comprising a spatially extended collection of dipoles 10 will have a reduced ability to transmit electromagnetic radiation along the axis DP1 of the dipoles 10, as will be further described below. Naturally, the radiation pattern as now described is equally valid for reception.
The individual antenna elements in an array antenna may also be the well-known monopole 20 or similar, as schematically illustrated in FIGS. 2A-2D. The exemplifying monopole 20 in FIG. 2A has a single radiating element 21 extending ¼ (λ/4) of the utilized wavelength from a substantially horizontal ground plane 23 and along a substantially vertical axis MP. In other words, the monopole 20 is a quarter-wave antenna or a so-called Marconi antenna. The radiating element 21 is connected to a feeding line (not shown in FIG. 2a-2d) in a well-known manner for communicating high frequency signals to and from the monopole 20, and the radiating element 21 is fed by a single unbalanced current I+ (not shown in FIG. 2a-2d) as is well-known in the art. Unbalanced single port monopole antennas like this have also been studied extensively.
FIG. 2B illustrates a cross-section of a schematic radiation pattern from the monopole 20 cut along the axis MP, and FIG. 2C illustrates a top-view of said schematic radiation pattern, whereas FIG. 2D illustrates a schematic perspective view of the radiation pattern in FIGS. 2B-2C. As can be seen there is substantially no radiation emanating along the axis MP, i.e. there is substantially no radiation emanating from the radiating element 21 along the normal to the ground plane 23. This implies that array antennas comprising a spatially extended collection of monopoles 20 will have a reduced ability to transmit electromagnetic radiation along the axis MP of the monopole, as will be further described below. Naturally, the radiation pattern as now described is also valid for reception.
The attention is now directed to a first exemplifying array antenna arrangement, illustrated in FIGS. 3A and 3B.
FIG. 3A is a schematic top view of an exemplifying array antenna 30 comprising an array of three dipoles 30a, 30b, 30c, e.g. such as the dipole 10 illustrated in FIGS. 1A-1D. The dipoles 30a-30c in FIG. 3A are collinearly arranged along an axis DP2 on the surface of a substantially flat substrate 33. As is well-known, the first dipole 30a has two radiating elements 31aa, 31ab, each connected to a feeding line 32aa, 32ab, whereas the second dipole 30b has two radiating elements 31ba, 31bb, each connected to a feeding line 32ba, 32bb and the third dipole 30c has two radiating elements 31ca, 31cb, each connected to a feeding line 32ca, 32cb. 
FIG. 3B is a schematic side view of the exemplifying array antenna 30 in FIG. 3A. As can be seen, the collinear radiating elements 31aa-31cb and the feeding lines 32aa-32cb are arranged on the surface of the substrate 33 so as to extend in the same or an adjacent plane. As is well-known, the direction of maximum radiation (the main lobe) of an antenna as the array antenna 30 in FIG. 3A-3B is perpendicular to the horizontal plane in which the radiating elements 31aa-31cb extend. This has been indicated in FIG. 3B by a first arrow 35 extending perpendicularly upwards from the substrate 33, and a second arrow 35′ extending perpendicularly downwards from the surface of the substrate 33. The second arrow 35′ has been drawn by dashed lines to indicate that the radiation in this direction may be attenuated, stopped or reflected by the substrate 33, i.a. depending on the composition of the material in the substrate 33.
The type of array antenna schematically illustrated in FIGS. 3A-3B is generally referred to as “broad side array” antennas, since the radiation originates predominately from the broadside of the array than from the end side. Scanning the main lobe 35 of the broadside antenna 30 is achieved in a well-known manner by prescribing a certain phase increment ψ between the antenna elements 30a, 30b, 30c in the scan direction Φ. Consequently, a first signal I+, I− with a first phase angle θ is feed to the first antenna element 30a; a second signal I+, I− with a second phase angle θ+ψ is fed to the second antenna element 30b and a third signal I+, I− with a third phase angle θ+2ψ is feed to the third antenna element 30c. The scanning itself is accomplished by varying the phase increment ψ, as is well-known in the art of phase controlled array antennas. The signals I+, I− mentioned above have been provided with opposite suffixes to indicate that they are out of phase by 180°, i.e. to indicate that the dipoles 30a-30c operate according to a balanced or differential mode in a well-known manner.
However, as the phase increment ψ increases so that the scan direction Φ of the main lobe 35 approaches 0°, i.e. approaches the horizontal direction in which the radiating elements 31aa-31cb extend, the impedance of the dipoles 30a-30c in the array antenna 30 changes in such a way that the matching deteriorates. This implies that an array antenna 30 comprising a spatially extended collection of dipoles 30a-30c or similar has a reduced ability to transmit electromagnetic radiation in directions that approaches the direction in which the radiating elements 31aa-31cb extend. In other words, there is substantially no radiation along the axis DP2, i.e. from the short ends of the radiating elements 31aa-31cb, which is consistent with the findings in connection with the single dipole 10 described above. Naturally, the radiation pattern as now described is also valid for reception.
The attention is now directed to a second exemplifying array antenna arrangement, illustrated in FIGS. 4A and 4B.
FIG. 4A is a schematic top view of an exemplifying array antenna 40 comprising an array of six monopoles 40a, 40b, 40c, 40d, 40e, 40f, e.g. such as the monopole 20 illustrated in FIGS. 2A-2D. Each monopole 40a-40f has a radiating element 41a-41f. The radiating elements 41a-41f are arranged in a straight line L1 on the surface of a flat ground plane 43. Each radiating element 41a-41f is furthermore connected to a feeding line 41a-41f in a well-known manner.
FIG. 4B is a schematic side view of the exemplifying array antenna 40 in FIG. 4A. The radiating elements 41a-41f extend from the surface of the ground plane 43 along vertical axes MPa-MPf, whereas the feeding lines 42a-42f are arranged in or adjacent to the ground plane 43. As is well-known, the possible directions of maximum radiation (the main lobes) of an antenna as the array antenna 40 extend along the line L1—i.e. along the line of radiating elements 41a-41f—and in parallel to the ground plane 43. This is indicated in FIG. 4B by a first arrow 45 to the right and a second arrow 45′ to the left.
The type of array antenna 40 schematically illustrated in FIGS. 4A-4B is generally referred to as an “end-fire array” antenna, since the radiation originates predominately from the end of the array and not predominately from the broadside of the array as in the broad-side array antenna 30 in FIGS. 3A-3B. Some scanning of the main lobe 45, 45′ of the end-fire array antenna 40 may be achieved in a well-known manner by prescribing a certain phase increment ψ between the antenna elements 40a-40f in the scan direction Φ. Consequently, a first signal I+ with a first phase angle θ can be feed to the first antenna element 40a; a second signal I+ with a second phase angle θ+ψ can be fed to the second antenna element 40b; a third signal I+with a third phase angle θ+2ψ can be feed to the third antenna element 40c, and so on to a sixth signal I+ with a sixth phase angle θ+5ψ that is feed to the sixth antenna element 40f. The scanning is then accomplished by varying the phase increment ψ, as is well-known in the art of phase controlled array antennas. The signal I+ have been provided with positive suffix to indicate that the signals fed to the monopole has the same original phase θ, i.e. to indicate that the monopoles 40a-40f operate according to an unbalanced or sum-mode in a well-known manner.
However, as the phase increment ψ increases so that the scan direction Φ of the main lobe 45 or 45′ approaches 90°, i.e. approaches the vertical direction in which the radiating elements 41a-41f extend, the impedance of the antenna elements 40a-40f in the array antenna 40 changes in such a way that the matching deteriorates. This implies that an array antenna 40 comprising a spatially extended collection of monopoles 40a-40f or similar has a reduced ability to transmit electromagnetic radiation in directions that approaches the vertical direction in which the radiating elements 41a-41f extend. In other words, there is substantially no radiation along the axes MPa-MPf of the radiating elements 41a-41f, i.e. along the normal to the ground plane, which is consistent with the findings in connection with the single monopole 20 described above. Naturally, the radiation pattern as now described is also valid for reception.
To summarize, the well-known dipole 10 and the well-known monopole 20 and variations thereof are frequently used as single antenna elements in array antennas, e.g. as in the broadside antenna 30 in FIGS. 3A-3B and in the end-fire antenna 40 in FIGS. 4A-4B. However, almost without exception the antenna lobe of these single antenna elements have formally at least one null point, i.e. at least one direction in which the antenna element cannot not readily transmit and receive electromagnetic radiation. It follows that an array antenna comprising a spatially extended collection of several such antenna elements is typically showing at least one direction in which the antenna lobe of the array antenna cannot be readily directed, i.e. there is at least one null point in the antenna diagram of an array antenna comprising such antenna elements.
Consequently there is a need for an improved array antenna and particularly an array antenna with improved ability to direct the antenna lobe, especially so as to reduce possible null points.